1. Technical Field
This invention relates to systems for optically determining the deviation in the curvature of a workpiece from a reference workpiece, and more particularly to a real time interferometric comparator for optically determining deviations in the curvature of a reflector, such as an antenna, from the curvature of a reference reflector.
2. Discussion
Current satellite frequency bands are being raised to provide new spectrum space due to the allocations of existing frequency bands and the required separation necessary to meet economical ground station beamwidth parameters. The extension into shorter wavelength, .lambda., communication systems has made necessary new measuring, manufacturing, testing and alignment techniques with respect to antennas, such as curved reflectors used with orbiting satellite systems, to achieve and maintain acceptable surface contour accuracies. Presently, surface contour accuracies in the range of about .lambda./50 to .lambda./100 represent a minimum acceptable range of surface contour accuracies. Many applications, however, require accuracies greater than .lambda./100.
Three characteristics of an antenna pattern are particularly important. The first is the intensity of the radiated pattern at the center of the antenna. This characteristic is typically known as "main-lobe gain". The main lobe gain of an antenna establishes how much power must be supplied to the antenna aperture in order to achieve an acceptable power density at the receiving element or, conversely, it determines the required strength of the transmitter signal at the radiating element to generate adequate field density at the aperture of the antenna. Radio frequency (RF) energy distributions in common reflector antenna systems are highly dependent upon the shape and RMS surface accuracy of the reflector.
Current techniques of measuring and adjusting antenna reflector surfaces of high performance reflector antennas operating in the millimeter wave region (30 GHz to 100 GHz) suffer from a number of drawbacks. Such current techniques are typically time consuming, expensive and yield results which are difficult to process, and which are further not provided in real time. Current techniques also often lack the accuracy to properly confirm the contour mapping of large, high performance reflector antennas.
One new technology that attempts to overcome the above-mentioned shortcomings involves the use of modulated, laser radar to determine the location and magnitude of irregularities in an antenna reflector surface. However, this technology requires high pixel residence times to gain the accuracy necessary to approach and maintain RMS surface contour accuracies up to .lambda./100. On large, short wavelength reflectors the number of pixels can range into the millions, thus increasing the measurement error of such systems.
In satellite applications, it is particularly important to be able to detect and correct errors in the curvature of irregularly curved reflector antennas in real time. Irregularities may be caused by changing thermal conditions which are brought on by an orbiting pattern of the satellite, such as when the satellite enters and leaves areas covered by shadows. Current systems and methods are presently unable to monitor changes in surface contours of irregularly shaped curved surfaces due to manufacture, test, alignment procedures and/or orbit conditions in near real time. Providing this capability could significantly reduce manufacturing costs associated with manufacturing large reflector antennas as well as virtually any other curved workpiece. This could also help to improve the accuracy of the surface of such a reflector antenna or other workpiece, in addition to providing a means of monitoring and correcting large reflector antenna surfaces while such antennas are on orbit.
Accordingly, it is a principal object of the present invention to provide a real time, interferometric comparator which is operable to optically monitor and detect irregularities in the curvature of a large, curved object such as a reflector antenna of a satellite, and to provide correction signals to the reflector antenna to enable controlled changes in the curvature of the antenna to be implemented, to thereby eliminate irregularities in the curvature in real time.
It is a further object of the present invention to provide a closed loop, real time interferometric comparator apparatus particularly well adapted to monitoring the curvature of curved reflector antennas of orbiting satellite systems and to provide highly precise corrections to such curved reflector antennas, in real time, to maintain a desired curvature for such reflector antennas.
It is yet another object of the present invention to provide a real time interferometric comparator apparatus which may be implemented using conventional interferometric technology to monitor the curvature of a reflector antenna.
It is still another object of the present invention to provide a real time interferometric comparator apparatus which may be implemented with conventional holographic imaging techniques for monitoring the curvature of large reflector antennas.
It is still another object of the present invention to provide a closed loop, real time interferometric comparator apparatus which may be used in manufacturing, test and alignment processes to provide an indication, in real time, of irregularities in the curvature of a large, curved workpiece, to thereby enable corrections to be applied to the workpiece, such as in manufacturing applications, in real time.